I received this question (not exact wording) on an assessment in relation to metabolism and the ETC. I found it odd, but it was a legitimate question with 6 marks if answered most accurately. I've given my own reasons but I would like to hear yours.

1. The heat our bodies generate is not enough to cause combustion.2. We don't have chemicals (as I know) in our body that can cause sudden combustion.3. We are more than 60% water- and like I stated, our body heat is not so much.

It would be interesting to calculate the energy released if all the body's nucleotide triphosphates (e.g. ATP) were to simultaneously hydrolyze. I see no mechanism for that happening, but that should represent a pretty good stock of potential energy.

There is the H+ leakage in mitochondria (brown adipose tissue). However, human body contains only small amount of ATP at a moment, although it burns like few kgs per day (it's produced and consumed all the time).

I'd say the point No. 3 is very important. If you have 70 kg person, he or she contains about 42 kg of water. The specific heat capacity of water is ~4200 J/kg.°C. If that person has temperature of 37°C (of course the peripherals will have lower temperature) and the autoignition temperature would be let's say 200°C (autoignition temperature of paper, which is the closest to human composition from the list, is over 200°C http://en.wikipedia.org/wiki/Autoignition_temperature ), that makes difference of 163°C. Thus one would need 163*42 = 6.8 kJ

A common explanation for SHC is a metabolic runaway or a ''short-circuit'' in the nervous system, liberating heat which causes a person to catch fire. What do you say to that?

PS. The 60% water argument is invalid. Radiators can cause things placed onto them to catch on fire, even though internally a radiator is merely hot water. Any heat will be conducted to the skin which will readily ignite as it contains little to no water.

Thank you Jack, I enjoyed riding along with your calculations. Coelacanth, your idea that the effect of water is lessened because only the skin is an issue for combustion is an interesting hypothesis. I'm not ready to expect spontaneous human combustion until I have seen reliable documentation, but the emerging argument makes a good tale .

1) your radiator argument is invalid. If something catches fire on the radiator, it's because the water is hot. While the water in your body has 37°C and must be heated up, which requires about 7 kJ, as I calculated above.2) as you said, he burned from inside out, i.e. the inwards must burn first, thus it's not question of skin only. 3) I told you about the H+ leakage in brown adipose tissue. But that tissue is made to produce heat. Other tissues prevent that from happening.

As far as I know, fires caused by radiators are caused by electric radiators, not water/oil filled ones. Especially the water-filled radiators rarely cause fires, since the water is typically far below 100C and even if it were boiling, the pipes would burst rather than cause fire anyway. Or am I missing something here, can you really ignite things with a hot water radiator?

I am not aware of any neural short-circuit mechanism that could cause fire, because a) the energies in the nervous system are so small that you probably could not ignite even gasoline with that and b) the human body lacks good material for burning, mostly because we are so "wet".

My best guess for something that could be mistaken as a spontaneous human combustion would be a discharge of static electricity on some highly flammable material that would then cause the person to catch fire as well. Though that would probably not be technically SHC.

I have always been under the impression that SHC is a persistent urban legend.